Some fats are good for us, others not so. But then there’s the star – intramuscular fat. Nicola Dennis explains the benefits of IMF.

Fat. It’s a little word that carries a lot of emotional baggage. One can scarcely mention the word without the internet offering a dozen public health guidelines and10 hidden secrets to shrink yourself. But when it comes to the eating quality of meat, fat is one of the good guys.

In fact, intramuscular fat may be the only fat the predatory “as seen on TV” advertisers don’t want us to lose. I took a look at the basics of meat science and, as it turns out, it’s not simply a matter of “fat tastes nice”.

There are many ways of classifying fat. One of the main ones is where you find the fatty tissue (i.e. adipose tissue). If it is wrapping around the internal organs inside the abdominal cavity, it is called abdominal or visceral fat. This is the kind of fat you would see on a beef heart or around the kidneys.

Its purpose, like all fatty tissues, is to store energy but it’s also a major regulator of endocrine signalling in your body (how the different parts of the body talk to each other). It’s also the type of fat your doctor is most worried about you accruing, because of its association with cardiovascular disease and diabetes.

Fear not, however, if you happen to be sporting a bit of a puku, it’s not necessarily abdominal fat. Your jolly belly fat might be just sitting under the skin as subcutaneous fat. This is the fat that keeps you warm and cushions you against the hard knocks of life.

Calm your itchy trigger finger and give your homekill animal some time to fill it’s intramuscular fat cells.

With meat, subcutaneous fat is the kind of fat you will see coating the side of a juicy lamb/beef/pork chop. If we look closer at our chop of choice we might also see thick streaks of white where different muscles meet. That is the intermuscular fat that sits between muscles, it acts as soft padding for the muscles as they move.

Then there is a more subtle kind of fat, intramuscular fat (IMF) which sits in small quantities inside the muscle ready to fuel muscle movements. This is the “marbling” consumers get excited about.

There are also specialised types of adipose tissue that live around the body doing things not at all related to meat, such as brown/beige fat (primarily a furnace for heat), bone marrow fat and brain fat.

I only add these for completeness. The intramuscular fat is the star of this show.

Why is IMF good?

When it comes to meat yumminess, there are three key requirements to check off – flavour, juiciness and tenderness. Increasing intramuscular fat improves all three of these aspects to a degree.

Fat imparts flavour – in fact fatty acids and the things packaged with them seem to be where many of the species-specific flavours live. So a higher quantity of IMF can help your beef taste beefy, your lamb tasty lamby and your billy goat taste like wet dog.

A higher fat content also helps with lubricating the mouth during eating, triggering more saliva and making the meat seem more juicy. Have I spoiled the illusion by pointing out the juiciness is your own spit? I sincerely hope not.

When it comes to tenderness and IMF, it seems replacing some of the muscle fibres (and the extracellular matrix that holds the muscle fibres together) with fat cells leads to a less dense, easier-to-eat steak.

Of course, this overlooks all the other chemistry going on inside the meat. Glucose, salts, free amino acids, pH, water-holding capacity, collagen content, collagen solubility, heat shock proteins etc all affect the flavour/juicy/tender checklist too.

The types of fatty acids packed into the fat cells matter too because of the aforementioned flavours and because fats with lower melting points will melt in your mouth for your full enjoyment..

Even then, all the stars can be in alignment, but you blow it all with cooking temperature.

It comes as a great surprise to me, an entirely unsophisticated chef who often can’t identify which species of homekill she has fished out of the freezer, even after eating it, that I should not fry everything on high and bake everything at 180C.

How much IMF is good?

The amount of intramuscular fat in your meat depends on which animal you have selected and which muscle you are chowing down on.

The percentage of IMF present can range from 1-2% for the venison shoulders, chicken breast sits at 3-4%, a lamb loin is about 5% and a high marbling score Wagyu beef steak can exceed 50%. I even found a Polish research group looking at IMF concentrations (0.96 to 1.4%) in various muscles of the European beaver.

Obviously that is a very wide range of values and all those meats taste juicy and tender in their own right (with the possible exception of the beaver). So, it is hard to say how much intramuscular fat cuts the mustard without going into the specifics of individual meat cuts. However, it would seem there is a point where the extra IMF isn’t doing much. For example, one research team in Australia who were looking

at beef striploins found that eating quality benefits plateaued once IMF got to 13-20%. The aforementioned Wagyu beef sits well above this threshold (typically in the 20-35% range).

There are multiple uncharitable ways that we could interpret this, that Australian palates are unrefined, or Wagyu beef marbling is overkill. But, perhaps it is best to recognise that Wagyu beef has been bred for its unique flavour profile (as well as the marbling) and the fat is where the Wagyu flavours are most likely to live.

It probably goes without saying, but the intramuscular fat content may not be a big deal for non-steak cuts which typically contain chunks of subcutaneous/intramuscular fat and are nurtured into a tender, juicy, flavourful state through roasting, stewing or grinding. That being said, consumers appear to be willing to part with cash for Wagyu mince so there is no accounting for marketing. Note: the author has not tried Wagyu mince. Offended Wagyu marketers are advised to send Wagyu samples to our esteemed editor for distribution.

How do we improve IMF?

Clearly there is a genetic element to intramuscular fat. The Wagyu started life as a cold-resistant draught animal that could pack Vitamin A away in its muscles in preparation for the long snowy winters. Since graduating to Japanese cuisine in the late 1800s, there have been huge gains in the amount of IMF in Wagyu beef.

The heritability of IMF varies widely depending, again, on which species and which muscle you are focusing on. And like all genetic traits, it also depends on how accurately you are measuring IMF and how well you are keeping track of the environmental (non-genetic) factors that might aid or hinder IMF.

Let’s look at how intramuscular fat comes to be so that we appreciate all the ways we might influence it. For simplicity, I will focus on cattle which is where a lot of IMF science effort has taken place. The fat cells (i.e. adipocytes) that will store the IMF are installed during gestation and early life.

By the time the calf is about 250 days old, it seems it has all the fat cells it is ever going to have anywhere in its body. In fact, for some of the faster-to-mature fat deposits, like abdominal fat, the die is cast when the calf is a newborn. So how well the calf’s mother was fed during pregnancy, her milk yield, the calf’s weaning age and weaning diet is likely to affect the number of IMF fat cells available.

Fat soluble vitamins like vitamin A and D can also be important, although not always in the way expected as some researchers postulate that restricting vitamin A may be favourable for overall IMF. Possibly not favourable for the animal.

Once the fat cells for IMF have been built, the only thing left to do is cram them full of the good stuff. This only happens when the animal has surplus energy to store, so the animal must be enjoying a diet in excess of its growth and maintenance needs. IMF fat cells have to wait at the back of the queue behind the other fatty tissues.

So IMF fat cramming usually occurs later in the finishing stage. Therefore, slaughter age and weight is also important. Calm your itchy trigger finger and give your homekill animal some time to fill it’s intramuscular fat cells. If your processor pays premiums for marbling, take care with your finishing liveweights to give the intramuscular fat a chance to develop. I’m looking at you, lower South Island. I know from my days as a market analyst, that you are routinely knocking the heads off under-grown stock.

Perhaps you could consider giving your light prime animals a bit of extra time (or investigate their store value with someone further up the country). You can only kill them once. They may as well go out big and juicy.

Winning trend

It’s looking like a win:win for fatty flavours and the environment. Results from AgResearch’s high and low methane sheep experiment show low methane emitting sheep have improved fatty acid profiles in their intramuscular fat.

This finding is not as outrageous as it might seem, because the acidic environment that creates methane in the rumen is also detrimental to the formation of fatty acids needed to make IMF.

Nicola Dennis is a scientist and data wrangler.

Note: the author has not tried Wagyu mince. Offended Wagyu marketers are advised to send Wagyu samples to our esteemed editor for distribution.